If the Firefox user interface (menu bar) is in the wrong language or if you want to change the current language then you can get Firefox in the language of your choice here:
* Firefox 5.0.x: http://www.mozilla.com/en-US/firefox/all.html

'''British and USA Versions'''
I downloaded the British Version of Firefox, and living in Canada, I would have chosen the US Version.
1) Does it Matter? Does it affect any settings, choices, abilities, or set up of the program.
2) Is there any difference between the two?
Link for US version http://www.mozilla.org/en-US/firefox/new/

If you do not need the en-US dictionary then you can install the en-GB Firefox version and install the Canadian dictionary from the Add-ons site.
*https://addons.mozilla.org/firefox/addon/canadian-english-dictionary/

For UK and other languages too ,
Just go to Firefox download see the big green button that is the US Firefox.
Now just look below it say's systems & Languages and click on this only . ( not the other two there ) . Now go down the list until you find English UK it is there then click on it to the side.
The first is Windows , then Mac , then Linux . Then auto updates should be in the language you have chosen.
Or click on this link should take you to the page ( if working )
http://www.mozilla.org/en-US/firefox/all.html'''
''''''